Depending upon their design and the heat sink used, rectifier diodes can be driven only up to a maximum current level. If this maximum current level is exceeded, the diodes are destroyed. Such destruction occurs not infrequently in battery-charging devices in motor vehicles, if the battery which is to be charged has its terminals connected incorrectly. In this case, the output diodes are switched in the on or forward biased direction, and a relatively high current flows through them, which generally destroys the diodes. Protecting the diodes by means of safety fuses has already been attempted. Safety fuses, however, require additional space, which runs counter to the need for miniaturization of components. Furthermore, holder means for safety fuses are unsuited to the unfavorable conditions in the engine area of an automobile. Electronic protective devices have also been developed; but such devices are too expensive for general application.